Innerseals are widely used throughout the packaging industry to cover the opening of a container and to provide a seal between the container and the cap. Generally, two types of innerseals are used. The first consists of a dimensionally stable substrate which may be placed over the opening of the container It is held in place only by the compressive forces between the container and the cap. As this type of innerseal is not adhered to the container, it does not protect against leaks and spills.
The second general type of innerseal utilizes an adhesive composition on one surface of a dimensionally stable substrate. The adhesive compositions employed most commonly are either pressure-sensitive compositions or heat-activated compositions While both of these compositions have been used to adhere innerseals to container openings, they both suffer from certain disadvantages.
Innerseals that employ pressure-sensitive adhesive layers commonly require the use of release liners to prevent the innerseal from adhering to itself prior to application to a container. This, in turn, makes handling and applying such innerseals more cumbersome and less efficient, and consequently, more expensive, as the liner must be removed and discarded before the innerseal can be used. Additionally, the liner creates waste which must be handled and discarded. Furthermore, many wet and dry products will adhere to the surface of the pressure-sensitive adhesive, resulting in the innerseals having a poor appearance.
Innerseals that employ heat-activated adhesive layers require the use of special application equipment, thereby adding to the expense of the packaging operation and increasing the opportunity for equipment malfunction. Furthermore, one of the more commonly used heat-activating techniques utilizes high frequency induction sealing. This technique requires the presence of a metal foil layer in the innerseal and, consequently, is useful for only certain types of innerseals.
Innerseal materials that can be adhered to a container by the application of shear forces have also been used. These materials utilize shear-activated adhesives based on block copolymers of styrene-ethylene-butylenestyrene to adhere the innerseal to the container. These materials are not, however, totally satisfactory as the low level of adhesion between the shear-activated adhesive and the lip of the container can be overcome by the pressure built up in the container by all liquid products and some dry products during filling operations, storage, and shipping. Moreover, some shear-activated adhesives cannot be used for fatty food products on account of FDA regulations.
The present invention overcomes the disadvantages of innerseals that utilize pressure-sensitive adhesives, heat-activated adhesives, and shear-activated adhesives. Even though the innerseal of the present invention is pressure-activated, it surprisingly adheres firmly to a container and provides a seal against leaks and spills therefrom even after the cap has been removed. Moreover, it is readily removable from the container. The innerseal of the invention may be applied to containers, portions of containers, laminates, coatings, or any substrate made from a variety of materials having high polarity, including polyvinyl chloride, polyester, polystyrene, polycarbonate, polyvinylidene chloride, and nitrile rubber or its derivatives, such as modified acrylonitrile-methacrylate copolymer.